The loop reactor bulk polymerization process for making propylene polymers is a well-known process, in which polymerization takes place in loop reactors with the polymer forming as solid particles suspended in a liquid comprising mainly propylene monomer. The reactor content is maintained in a highly agitated state by circulating the reaction mixture containing reactants and polymer at comparatively high velocity around the reactor loop by means of a circulation pump. The heat of polymerization is removed by a water cooling jacket enclosing each reactor leg. The polymer is removed from the reactor by opening and closing continuously or periodically a discharge valve to release both liquid and solid reaction mixture.
There are at least three major problems in the conventional loop reactor polymerization. The reactor temperature and pressure must be such that the entire reactor is completely filled with a reaction mixture containing reactants and polymer and no vapor bubbles will cause cavitation in the circulation pump. The reactor diluent must be selected to provide the minimum polymer solubility to prevent reduction of heat removal by fouling, especially with copolymers. The reactor diluent must be volatile to minimize the requirement for heat addition in order to separate diluent from the finished polymer powder.
In ethylene polymerization these problems can be avoided if supercritical conditions are used. The normally used diluent, isobutane, is replaced by propane and no bubble formation and cavitation in the loop reactor takes place, when the operating temperature and pressure are in the supercritical region. Further, the polymer is less soluble in supercritical propane and the flashing of monomer is easier, because the fluid contains more heat from the polymerization reactor. (see WO 92/12181). However, in ethylene polymerization with a slurry process the monomer concentration in the reaction medium is usually rather low (e.g. 5-15 wt. %).
Another known process for manufacturing propylene polymers is the gas phase process, where the propylene monomer is polymerized in a fluidized bed of forming polymer particles. The polymerization bed is cooled by circulating gaseous mixture of propylene and optional comonomers and hydrogen from the top of the reactor via cooler and returning it back to the bottom of the reactor. Make-up propylene is added to the circulation.
Known are also multistage processes for manufacturing polypropylene, where more than one reactor are used in series.
All known processes have some limitations. Therefore a need exists for a flexible process, where it is possible to produce various kinds of propylene polymers having a molecular weight range from high to low and molecular weight distribution from narrow to broad.
Propylene has a critical temperature, which is even lower than propane (91.4.degree. C./96.8.degree. C.). The polymerization of propylene under supercritical conditions is not novel. In U.S. Pat. No. 4,740,550 there is disclosed a multistage copolymerization process for making propylene-ethylene copolymers. In this process propylene is homopolymerized in one or more pipe-loop reactors, in the presence of magnesium and titanium containing catalyst, resulting in a homopolymer reaction suspension, transferring said homopolymer reaction suspension to a separator and separating a stream of reduced-fine homopolymer particles, transferring said stream of reduced-fine homopolymer particles to a gas-phase a fluidized bed homopolymer reactor and homopolymerizing additional propylene, removing a stream of propylene homopolymer particles and transferring it to a gas-phase fluidized bed copolymer reactor, and copolymerizing ethylene and propylene resulting in a propylene-ethylene impact copolymer.
In said patent, the homopolymerization of propylene under supercritical state in the pipe-loop reactor is mentioned as one alternative. The patent is theoretical and contains no real examples. The only calculated examples refer to the polymerization under supercritical conditions. A six-inch diameter pipe-loop reactor having a considerable length of about 400 m is described. The reactor has a total volume of 6.78 m.sup.3, which is a typical size of a prepolymerization reactor, and small when compared to ordinary loop reactors (35 m.sup.3). In such a reactor the residence time is only, 10 minutes. Otherwise, it would be impossible to circulate the polymer reaction medium in the pipe reactor. Moreover, the process described in the patent is rather complex, comprising three reactors and is intended to produce propylene-ethylene impact copolymers.